1. Field of the Invention
This invention relates in general to a magnetic write circuit and, more specifically, to a circuit for writing digital input signals onto magnetic media such as disks.
2. Background Art
In contemporary data processing systems, data is stored on magnetic tape or magnetic discs for retrieval and use at a later time. The data is recorded on one or more tracks of the magnetic medium in the form of a magnetic pattern, i.e., a sequence of ones and zeros, which corresponds to a digital input signal pattern. Conventional circuitry for writing a digital input signal pattern onto a magnetic media comprises an induction coil coupled to a circuit responsive to the digital input signal. A typical magnetic write circuit is shown in FIG. 1 and comprises flip-flop 11 coupled to receive the digital input signal on terminal 12. Differentially connected NPN transistors 13 and 14 have their emitters coupled to supply voltage terminal 15 by current source 16. The base of transistor 14 is connected to output Q of flip-flop 11, and the base of transistor 13 is connected to output Q of flip-flop 11. Outputs Q and Q are logically complementary signals of one another. Induction coil 17 is coupled between the collectors of transistors 13 and 14, and is coupled to voltage supply terminal 18 in a manner known to those skilled in the art. Coil 17 is part of a magnetic head (not shown) in which a magnetic disk (not shown) may be rotated. Digital input data is applied on terminal 12 to flip-flop 11, which alternately switches transistors 13 and 14 on and off. Collector currents I.sub.13 and I.sub.14 of transistors 13 and 14, respectively, flow through coil 17, thereby generating magnetic flux within the magnetic head for "writing" on the disk. Magnetic particles on the disk correspond to the digital input signal.
FIG. 2 illustrates waveforms associated with the previously known circuit of FIG. 1. At every pulse of the digital input data, Q and Q change state. When Q goes high, current I.sub.14 flows, and when Q goes high, current I.sub.13 flows. This results in the magnetized pattern and voltage V.sub.14 at the collector of transistor 14 as shown. The induced collector voltage V.sub.14 is generated in the coil when current flow into the coil changes. This induced voltage depends on inductance of the coil, amount of current change, speed of current change, stray capacitance and other factors.
When this induced voltage is restricted, i.e., V.sub.14.V.sub.CC for a 5 volt supply voltage V.sub.CC, V.sub.14 cannot reach a sufficient amplitude because transistors 13 and 14 saturate, and the collector current will be decreased. This reduced voltage V.sub.14 delays current flow. This problem is illustrated in FIG. 3. Current I.sub.14 is seen to lag current I.sub.13, resulting in a gap in the magnetic pattern.
The induced voltage swing in present devices is approximately four to seven volts while typical supply voltage V.sub.CC is twelve volts for five inch floppy disk drives and twenty four volts for eight inch floppy disk drives. It is desirable to achieve a five volt single supply floppy disk driver or a battery-operated floppy disk driver, wherein both have sufficient voltage margin at the coil to prevent transistors 13 and 14 from saturating. In both cases, low voltage write operation is essential.
Thus, a magnetic write circuit for low voltage disk drives is needed.